1. Field of the Invention
The present invention relates to a microcrystal silicon film used as a component of a solar cell and a thin-film transistor, and to a manufacturing method of such a microcrystal silicon film.
2. Description of the Related Art
Microcrystal silicon is known as a material which exhibits intermediate properties between amorphous silicon and single crystal silicon. As is known from Japanese Examined Patent Publication No. Hei. 3-8102 and Japanese Unexamined Patent Publication No. Sho. 57-67020, a plasma CVD method is known as a manufacturing method of a microcrystal silicon film. In this method, a microcrystal silicon film is deposited on a substrate by decomposing a mixed gas of a silane gas and a hydrogen gas by glow discharge. This method is characterized in that the mixed gas that is supplied to a reaction space for forming a film should be composed such that the amount of hydrogen gas is tens to hundreds of times larger than the amount of silane gas, and in that glow discharge is caused by inputting electric power at a high density. Further, if a diborane gas, a phosphine gas, or the like is added to the above mixed gas for the purpose of valence electron control, doping is effected so efficiently that there can be obtained a high electric conductivity which cannot be attained by an amorphous silicon film. For this reason, a microcrystal silicon film is frequently used as a valence-electron-controlled doped layer, i.e., a p-type or n-type layer to constitute a photocell or a thin-film transistor.
In manufacture of a microcrystal silicon film in which a silane material gas is diluted, the film forming rate is substantially determined by the silane gas supply amount and is lower than that of an amorphous silicon film. The film forming rate of a microcrystal silicon film is approximately in a range of 0.01-0.1 nm/s. A film forming rate lower than this range is not practical, whereas a microcrystal silicon film is not formed at a film forming rate higher than this range.
To increase the film forming rate, techniques for increasing the density of a silane gas or the input discharge power would be conceivable. However, the range of conditions which allows successful formation of a microcrystal silicon film is restricted; under the conditions out of that range, the crystal grain diameter of a film formed becomes too small, and reduction in crystal density prevents formation of a high-quality microcrystal silicon film.
The valence electron control of a microcrystal silicon film can be performed to obtain a film of p-type or n-type conductivity by adding an impurity during the film formation by using a doping gas of diborane, phosphine, or the like. It is an empirical fact that the addition of diborane, among those doping gases, particularly makes it difficult to effect microcrystallization.
The microcrystal silicon film is applied to the solar cell to form a p-type or n-type layer. To reduce the light absorption loss, those layers are made as thin as about 10-50 nm at most. However, in forming such a thin microcrystal silicon film, the interaction with an undercoat material prevents sufficient microcrystallization.
For example, in forming a solar cell having a PIN junction. a heterojunction is formed by depositing a p-type layer of about 10 nm in thickness on an i-type amorphous silicon film. However, the deposition of a microcrystal film on an amorphous film causes lattice distortion, so that sufficient microcrystallization is not effected at the initial stage of the deposition and amorphous components become dominant in the corresponding region of a film formed. Thus. microcrystal silicon layers of solar cells formed according to the conventional techniques not necessarily have sufficient characteristics.
Although it is possible to produce a solar cell in which the entire PIN junction is made of microcrystal silicon, in this case the thickness of the i-type layer should be about 1,000 nm or preferably more than 1,000 nm due to the optical properties of the microcrystal silicon films. However, since the film forming rate of a microcrystalline silicon film is low, this type of configuration is not practical. For example, under film forming conditions for 0.03 nm/s. which is a typical film forming rate of a microcrystal silicon film, it takes more than 9 hours to deposit a 1,000-nm-thick film. This kind of process is extremely low in practicality.
An object of the present invention is therefore to form a high-quality microcrystal silicon film that is superior in crystallinity while increasing its film forming rate.
Another object of the invention is to form a microcrystal silicon film that is superior in crystallinity in thin-film devices such as a thin-film transistor or as a p-type or n-type layer of a solar cell.
To attain the above objects, according to the invention, to form a microcrystal silicon film that is better in quality than conventional microcrystal silicon films by using the conventional plasma CVD as a basis, a metal element for accelerating crystallization of silicon is added during the film formation as a means for accelerating microcrystallization of the film.
The metal element may be one or a plurality of elements selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au. In particular, very favorable results can be obtained by using Ni due to its large effects and high reproducibility.
The use of the above-described metal element facilitates microcrystallization and improves the film forming rate. As a result, a high-quality microcrystal silicon film having superior crystallinity can be obtained even if it is as thin as about 10 nm.
Where nickel is used as the metal element, nickel is introduced into a deposited film by adding, to material gases, a gas of a compound including nickel as a main constituent by using the conventional plasma CVD technique as a basis. It is proper that the nickel concentration be 5xc3x971016 to 5xc3x971019 cmxe2x88x923. No marked effects are observed if the nickel concentration is lower than this range, and the film characteristics become worse if it is higher than the above range.
Another method of adding nickel to a film is such that by likewise using the conventional plasma CVD technique as a basis. a nickel filament is disposed in a glow discharge space and heated during the film formation.
The invention can be applied not only to a solar cell but also. in principle, to photoelectric conversion devices, such as a photosensor, having similar functions as typified by the function of converting light to electrical energy.
If a metal element for accelerating microcrystallization of silicon is added to reaction gases during deposition of a microcrystal silicon film by plasma CVD, the metal element serves as nuclei of crystal growth, thereby facilitating the microcrystallization as compared to the case of not adding the metal element. The microcrystallization occurs from the initial stage where a film being deposited is still very thin. With the metal element serving as nuclei of crystal growth, the film forming rate of a microcrystal silicon film can be increased easily.
As for the electrical characteristics of a film, a film having improved crystallinity can be effectively doped in performing valence electron control to obtain, for instance, p-type or n-type conductivity. whereby the film is given a lower resistance than in the conventional case. Further, electrical characteristics equivalent to those of a conventional film can be attained even with a thinner film.
The above features are advantageous in a microcrystal silicon film that is used as a p-type or n-type layer of a solar cell. These layers are usually formed at a thickness of 10 to 50 nm at least, but the conventional techniques cannot provide a sufficiently high degree of crystallinity in such a thickness range. In contrast, the manufacturing method of the invention greatly improves the crystallinity. With this advantage, a p-type or n-type microcrystal silicon layer that is formed on the light-incidence-side of a solar cell can be made sufficiently thin.